feat: update.href api (#6248)

add `update.href` property option to update objects send via
`Context::send_webxdc_status_update()`.

when set together with `update.info`,
UI can implement the info message as a link that is passed to the webxdc
via `window.location.href`.
for that purpose, UI will read the link back from
`Message::get_webxdc_href()`.

Practically,
this allows e.g. an calendar.xdc
to emits clickable update messages
opening the calendar at the correct date.

closes #6219

documentation at https://github.com/webxdc/website/pull/90

src/debug_logging.rs

@@ -60,6 +60,7 @@ pub async fn debug_logging_loop(context: &Context, events: Receiver<DebugEventLo
                         "time": time,
                     }),
                     info: None,
+                    href: None,
                     summary: None,
                     document: None,
                     uid: None,